This invention pertains to a topping arrangement of two or more steam turbine plants with a gas turbine.
Combined cycle units with heat recovery steam generators (HRSG's) as well as combined cycle plants with fully-fired steam generators are known. As a result, a variety of combined cycle arrangements based on simple plant power concepts have been developed. Thus, combined cycle plants with fully fired steam generators have been in operation as early as 1965. The gas turbines for these plant concepts are equipped with a heat recovery steam generator or heat exchanger utilizing the gas turbine exhaust energy to provide additional main steam, reheat steam or feedwater heating for steam plants, thereby increasing their output and overall power plant efficiency. These power plant concepts designated as compound cycles can be adopted for new facilities, but are especially suitable for repowering or topping projects. Since only a minimum of new steam plant equipment is needed, these plant concepts can become attractive low cost projects. See Maghon, H., Bermann, D., Bruckner, H., Kriesten, W., and Termuehlen. "Combined Cycle Power Plants for Load Cycling Duties" American Power Conference, Chicago, Ill., April 1989; Kreutzer, A., Ganzer, W., and Termuehlen, H., "Gas and Coal-Fired Combined Cycle Plants" American Power Conference, Chicago, Ill., April 1986; and Denizci, H., and Hamann, B., "Design and Operation of Ambarli Combined Cycle Power Plant" AEIC, Committee on Power Generation, Sep. 1991.
The largest plant built with a fully fired steam generator, also known as a "hot wind box" design is in Germany. It features four 417 MW size units and one 700 MW unit providing a net plant output of 2300 MW. The 770 MW unit features a coal-fired steam generator with a desulfurization system. Since the reliability of the previously installed gas turbines has been outstanding, it was decided to provide only 60% forced draft (FD) fan capacity for the operation of the steam plant without gas turbine, reducing the output for this mode of operation from 656 MW to roughly 500 MW. In the normal combined cycle operating mode, the gas turbine exhaust is supplied as preheated air with about 16% oxygen to the steam generator and its coal mills. A cooling air fan provides air to control the temperature in the mills and for FD-fan operation a primary air heater is installed. A partial-flow economizer provides feedwater heating in parallel to the HP feedwater heaters of the steam plant.
The fully-fired concept has also been applied for repowering a 590 MW power station. See Maghon, H., Schulenburg, T., Laakkonen, M., Froehlich, G., and Termuehlen H., "Full-Load Testing of the Advanced V64.3 Gas Turbine" American Power Conference, Chicago, Ill. April 1991. A V94.2 gas turbine has been installed and an auxiliary FD-fan was provided to achieve maximum output in the combined cycle operating mode with the two original larger FD-fans being only used for back-up operation, without the gas turbine. A flue gas bypass is provided for partial load operation of the steam plant to reduce the hot air supply to the furnace. HP and LP partial flow economizers are used in parallel to the HP and LP feedwater heaters to further improve the overall plant efficiency which was measured to be 46.6% or 7320 Btu/kWh at rated output. Comparing this performance with the original reheat steam plant efficiency of 40.7% (8,380 Btu/kWh) reveals a plant performance improvement of 5.9 percent points or 1,060 Btu/kWh. Load cycling operation between 100% and 45% plant output can be performed without plant efficiency deterioration. The NO.sub.x emission of the power plant was reduced to 30% of the original level from 400-500 ppm to 100-150 ppm at 3% oxygen content of the steam generator's stack flue gas.